This invention pertains to the analysis of an oil bearing core that has been subjected to an oil base drilling fluid.
An oil productive subsurface formation typically contains varying amounts of oil, gas and water in a porous matrix. Sometimes, it is important to determine the amount of oil present in order that various oil recovery techniques may be evaluated. For this and other purposes, cores are frequently taken to determine the amount of oil in the formation. The fluids are removed from the core and this pulse several measurements enables the oil saturation to be determined.
Sometimes it is desirable to use an oil base drilling fluid. For example, oil base drilling fluids have advantages in an unstable formation, or at higher temperatures, or when hydrogen sulfide is present. The typical oil base drilling fluid is comprised of an oil and chemicals and solids designed to control formation pressures, suspend drill cuttings and reduce fluid loss to the formation. The drilling fluid is usually at a higher pressure, for example, a couple of hundred pounds per square inch, than the formations which are subjected to the drilling fluid. Under certain conditions, the oil in the oil base drilling fluid invades the formation changing the oil concentration of the formation in the vicinity of the bore hole. When oil leaks in this fashion into the formation, the solids in the drilling fluid filter out on the face of the formation and the filter cake should stop further invasion of oil from the drilling fluid.
When a core is analyzed, it is important to know if the oil in the core has been significantly invaded by filtrate oil from the drilling fluid. It is standard practice to use a tracer chemical in a drilling fluid when it is important that drilling fluid invasion be determined. Tracer materials must be tested to determine their effects on drilling fluids and chemicals, their reaction with gases, their purity, their absorption characteristics on the filter cake or in the formation, solubility, and effects on drilling equipment and related facilities. Tracer materials are costly and frequently hazardous to drilling and core handling personnel. Moreover, the well that is being cored may be in a remote area. The drawbacks of tracer materials and the need for an improved system for detecting appreciable oil filtrate invasion has long existed.
In this invention, noticeable oil filtrate invasion is determined without the use of a separate tracer chemical. These and other advantages will be apparent from the following disclosure and appended claims.